Chang-Ki Moon

A Fluorescent Organic Light‐Emitting Diode with 30% External Quantum Efficiency

Almost 100% internal quantum efficiency (IQE) is achieved with a green fluorescent organic light-emitting diode (OLED) exhibiting 30% external quantum efficiency (EQE). The OLED comprises an exciplex-forming co-host system doped with a fluorescent dye that has a strong delayed fluorescence as a result of reverse intersystem crossing (RISC); the exciplex-forming co-hosts stimulate energy transfer and charge balance in the system. The orientation of the transition dipole moment of the fluorescent dye is shown to have an influence on the EQE of the device.

Highly Efficient Organic Light‐Emitting Diodes with Phosphorescent Emitters Having High Quantum Yield and Horizontal Orientation of Transition Dipole Moments

also used as the co-host of the emitting layer (EML) with the molar ratio of 1:1 to exploit the exciplex forming character for low driving voltage, and good electron-hole balance. [ 7,9,10 ] The detailed structure of the OLEDs is; indium tin oxide (ITO) (70 nm)/TAPC (75 nm)/TCTA (10 nm)/TCTA:B3PYMPM: 8.4 mol% dye (30 nm)/B3PYMPM (45 nm)/LiF (0.7 nm)/Al (100 nm). The photoluminescence (PL) spectra of the TCTA:B3PYMPM co-host fi lms doped with the three dyes with the doping concentration of 8.4 mol% are shown in Figure 2 a. The peak wavelength of the PL of Ir(ppy) 3 , Ir(ppy) 2 acac, and Ir(ppy) 2 tmd were 513 nm, 520 nm, 524 nm, respectively. The orientations of the transition dipole moments of the dyes in the host were determined through the analysis of the angledependent PL spectra of the fi lms. [ 7,11 ] Figure 2 b shows the measured angle-dependent PL intensities of the p-polarized light emitted from the 30 nm-thick fi lms composed of TCTA:B3PYMPM:green dyes (0.46:0.46:0.08 molar ratio) at 520 nm close to the PL maxima of the green dyes. The angledependent PL spectra were analyzed using the classical dipole model where the emission from excitons is considered as the dissipated power from oscillating dipoles. [ 12,13 ] Birefringence

Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes

Organic light-emitting diodes (OLEDs) are among the most promising organic semiconductor devices. The recently reported external quantum efficiencies (EQEs) of 29-30% for green and blue phosphorescent OLEDs are considered to be near the limit for isotropically oriented iridium complexes. The preferred orientation of transition dipole moments has not been thoroughly considered for phosphorescent OLEDs because of the lack of an apparent driving force for a molecular arrangement in all but a few cases, even though horizontally oriented transition dipoles can result in efficiencies of over 30%. Here we use quantum chemical calculations to show that the preferred orientation of the transition dipole moments of heteroleptic iridium complexes (HICs) in OLEDs originates from the preferred direction of the HIC triplet transition dipole moments and the strong supramolecular arrangement within the co-host environment. We also demonstrate an unprecedentedly high EQE of 35.6% when using HICs with phosphorescent transition dipole moments oriented in the horizontal direction.

Highly Enhanced Light Extraction from Surface Plasmonic Loss Minimized Organic Light‐Emitting Diodes

Extremely high light out-coupling efficiency from a transparent organic light-emitting diode integrated with microstructures on both sides of the device is reported. The metal free device offers dramatically reduced surface plasmonic and intrinsic absorption losses. Moreover, high refractive index micropatterns with optimal light extraction condition are fabricated based on the well-matched analysis of optical simulations.

Sky‐Blue Phosphorescent OLEDs with 34.1% External Quantum Efficiency Using a Low Refractive Index Electron Transporting Layer

Blue-phosphorescent organic light-emitting diodes (OLEDs) with 34.1% external quantum efficiency (EQE) and 79.6 lm W(-1) are demonstrated using a hole-transporting layer and electron-transporting layer with low refractive index values. Using optical simulations, it is predicted that outcoupling efficiencies with EQEs > 60% can be achieved if organic layers with a refractive index of 1.5 are used for OLEDs.

Highly Efficient Sky-Blue Fluorescent Organic Light Emitting Diode Based on Mixed Cohost System for Thermally Activated Delayed Fluorescence Emitter (2CzPN)

The mixed cohosts of 1,3-bis(N-carbazolyl)benzene and 2,8-bis(diphenylphosphoryl)dibenzothiophene have been developed for a highly efficient blue fluorescent oragnic light emitting diode (OLED) doped with a thermally activated delayed fluorescence (TADF) emitter [4,5-di (9H-carbazol-9-yl) phthalonitrile (2CzPN)]. We have demonstrated one of the highest external quantum efficiency of 21.8% in blue fluorescent OLEDs, which is identical to the theoretically achievable maximum electroluminescence efficiency using the emitter. Interestingly, the efficiency roll-off is large even under the excellent charge balance in the device and almost the same as the single host based devices, indicating that the efficiency roll-off in 2CzPN based TADF host is related to the material characteristics, such as low reverse intesystem crossing rate rather than charge imbalance.

Formation of perfect ohmic contact at indium tin oxide/N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine interface using ReO3.

A perfect ohmic contact is formed at the interface of indium tin oxide (ITO) and N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) using ReO3 as the interfacial layer. The hole injection efficiency is close to 100% at the interface, which is much higher than those for interfacial layers of 1,4,5,8,9,11-hexaazatripheylene hexacarbonitrile (HAT-CN) and MoO3. Interestingly, the ReO3 and MoO3 interfacial layers result in the same hole injection barrier, ≈0.4 eV, to NPB, indicating that the Fermi level is pinned to the NPB polaron energy level. However, a significant difference is observed in the generated charge density in the NPB layer near the interfacial layer/NPB interface, indicating that charge generation at the interface plays an important role in forming the ohmic contact.

Luminescence from oriented emitting dipoles in a birefringent medium.

We present an optical model to describe the luminescence from oriented emitting dipoles in a birefringent medium and validate the theoretical model through its applications to a dye doped organic thin film and organic light emitting diodes (OLEDs). We demonstrate that the optical birefringence affects not only far-field radiation characteristics such as the angle-dependent emission spectrum and intensity from the thin film and OLEDs, but also the outcoupling efficiency of OLEDs. The orientation of emitting dipoles in a birefringent medium is successfully analyzed from the far-field radiation pattern of a thin film using the model. In addition, the birefringent model presented here provides a precise analysis of the angle-dependent EL spectra and efficiencies of OLEDs with the determined emitting dipole orientation.

Highly efficient inverted top emitting organic light emitting diodes using a transparent top electrode with color stability on viewing angle

We report a highly efficient phosphorescent green inverted top emitting organic light emitting diode with excellent color stability by using the 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile/indium zinc oxide top electrode and bis(2-phenylpyridine)iridium(III) acetylacetonate as the emitter in an exciplex forming co-host system. The device shows a high external quantum efficiency of 23.4% at 1000 cd/m2 corresponding to a current efficiency of 110 cd/A, low efficiency roll-off with 21% at 10 000 cd/m2 and low turn on voltage of 2.4 V. Especially, the device showed very small color change with the variation of Δx = 0.02, Δy = 0.02 in the CIE 1931 coordinates as the viewing angle changes from 0° to 60°. The performance of the device is superior to that of the metal/metal cavity structured device.

Efficient Vacuum-Deposited Ternary Organic Solar Cells with Broad Absorption, Energy Transfer, and Enhanced Hole Mobility.

The use of multiple donors in an active layer is an effective way to boost the efficiency of organic solar cells by broadening their absorption window. Here, we report an efficient vacuum-deposited ternary organic photovoltaic (OPV) using two donors, 2-((2-(5-(4-(diphenylamino)phenyl)thieno[3,2-b]thiophen-2-yl)thiazol-5-yl)methylene)malononitrile (DTTz) for visible absorption and 2-((7-(5-(dip-tolylamino)thiophen-2-yl)benzo[c]-[1,2,5]thiadiazol-4-yl)methylene)malononitrile (DTDCTB) for near-infrared absorption, codeposited with C70 in the ternary layer. The ternary device achieved a power conversion efficiency of 8.02%, which is 23% higher than that of binary OPVs. This enhancement is the result of incorporating two donors with complementary absorption covering wavelengths of 350 to 900 nm with higher hole mobility in the ternary layer than that of binary layers consisting of one donor and C70, combined with energy transfer from the donor with lower hole mobility (DTTz) to that with higher mobility (DTDCTB). This structure fulfills all the requirements for efficient ternary OPVs.